I want to use different (JDBC) job repositories in one JBoss application server to separate jobs for different tenants.; my plan is that the jobs for every tenant are packaged into its own WAR package and deployed to the same JBoss.

In JBoss using admin console I can configure multiple JOB repositories (memory or JDBC) but I am not sure how I can tell some Job (specified using the xml job language and contained in a WAR) to use a specific job respository?

In admin console, you can create JDBC job repositories, and assign one of them as the default batch job repository (say, jdbc1) for the entire application server. It means all deployed batch applications will by default use this job repository (jdbc1).

You can also configure a custom job repository for each application via jboss-all.xml to override the default server-wide job repository. Please see the batch subsystem schema in your application server for more details.